1. Field of the Invention
The present invention relates to a telescope including an automatic focus detecting apparatus which can be applied to a surveying instrument such as an auto-level, etc.
2. Description of the Related Art
The surveying instrument basically comprises a collimating telescope. The collimating telescope is provided with a focusing lens group in order to view a clear object image regardless of the distance of the aiming object. The telescope is provided, in order from the object side, with an objective lens system (which includes the focusing lens group), and an eyepiece lens group system. The position of the focusing lens group is adjusted according to the distance from the object, so that an image of the object may be formed on a focusing plate. The operator may thus observe the image superimposed on the focusing plate via the eyepiece lens group system.
For example, provided that the range of the object distance is between 2 m-infinity, and that the focusing lens group is a concave lens, the approximate amount of movement of the focusing lens is about 30 mm. The focusing lens group is normally driven through an operation of a rotation knob. If the amount of movement of the object image (i.e., the amount of movement of the focusing lens group) is set smaller in regard to the rotating angle of the rotation knob, the object image remains on the focusing plate longer due to the smaller amount of movement of the object image when compared with the rotating angle, but it is time-consuming to move such a lens. On the other hand, if the amount of movement of the object image is set larger with respect to the rotating angle of the rotation knob, the amount of movement of the object image becomes too large when compared with the rotative angle, and the object image remains on the focusing plate for a shorter time. Consequently, the operator may sometimes pass by the focal point (in-focus condition) without noticing the coincidence of the object image with the focusing plate and the focal point, leading to extra time taken for refocusing.
Additionally, if the object is positioned far from the telescope, focusing is easily performed through only a slight operation of the rotation knob in the rotating direction, since the amount of movement of the object image is larger when compared with the rotative angle of the rotative knob. On the other hand, if the object is positioned relatively close to the focusing lens, the amount of movement of the object image is smaller when compared with the amount of rotation of the rotation knob, and it is difficult to move the object image on the focusing plate despite more rotation of the rotation knob. In such a case, it is difficult to determine whether the aiming object is in a front focus or a rear focus position, which may lead the operator to rotate the rotation knob in the wrong direction with respect to the focal point. In any event, the conventional surveying instrument suffers from the fact that it takes too much time for focusing.
An auto-level basically comprises a collimating telescope and an optical horizontal compensation system. The collimating telescope is substantially set, with respect to the horizon, then a reference object is observed. At that time, if an optical axis of the telescope is not perfectly horizontally set, the horizontal compensation system coincides the collimation line with a horizontal line. The telescope is then pivotally rotated around a vertical axis perpendicular to the optical axis, and when an operator views any collimation point, the viewed collimation point is positioned with the imaginary horizontal surface that includes the reference point (the set horizon).
In order to overcome the above-mentioned problem, an auto-level is provided, to which a TTL (through-the-lens) type of automatic focus detecting apparatus is mounted. As is well known, however, the auto-level requires a regular readjustment of the optical horizontal compensation system in order to maintain horizontal accuracy. The optical horizontal compensation system is decoupled from the auto-level during readjustment, and is coupled again after the readjustment ends. Therefore, when the automatic focus detecting system is mounted to the auto-level, easy decoupling and coupling of the optical horizontal compensation system is desired.